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New Twist on LCD Displays

New Twist on LCD Displays

A number of companies are working on developing alternative technologies to enable the production of flexible displays, including reflective "e-paper" and emissive organic light-emitting diode (OLED) technologies.

Over the next 10 years, thin-film polymers and other flexible substrates could change how people think about and use displays. In the future, you may "print out" reports to sheets of e-paper: flexible polymer displays about as thick as a sheet of paper that can be spread out on a desk for easy comparison and analysis and then reused when the work is done. Your PDA or cell phone may incorporate a roll-up display that extends to let you view maps or Web pages on a larger screen. Your laptop may have a secondary display on the back of the case that can maintain any image you choose, such as your schedule and to-do list -- and you'll be able to refer to it even when the laptop is turned off. Some displays may be embedded on a shirt sleeve or curve around a watchband.

"We're talking about electronics we can wrap around a pencil," says Jim Brug, imaging materials department manager at HP Laboratories. He says he expects such technologies to evolve into real product designs within five years.

While today's flexible display prototypes are relatively small, Hewlett-Packard and many other companies are working on flexible screens that measure as large as 14 inches diagonally. But the goal is to complement current LCDs rather than provide a substitute for them. "It's a mistake to think of this as a replacement for a [desktop or laptop] display," says Brug. "This is really a new way of using surfaces for information." That might include interlocking flexible panels, like sheets of wallpaper, to create a single, wall-size screen, he says.

Making traditional displays flexible presents several challenges. An active- matrix LCD consists of two layers of glass with several components in between: a thin-film transistor (TFT) layer embedded in amorphous silicon and etched onto the bottom glass, which produces the light pixels, and a liquid-crystal layer on top that acts as a light shutter. A backlight sits beneath the display, while a color filer and polarizers sit above the LCD. Creating a flexible display involves eliminating the backlight and replacing the glass layers with a flexible substance such as a thin polymer film.

The problem is that the liquid crystals in LCDs don't like to bend. "The quality of the image depends on the cell gap" between the polymer layers, says Kimberly Allen, director of display technology and strategy at iSuppli. The LCD will distort the image if the gap between the two layers isn't uniform when the substrate flexes. Also, an LCD with contoured surfaces can be difficult to view because of the angle.

A number of companies are working on developing alternative technologies to enable the production of flexible displays, including reflective "e-paper" and emissive organic light-emitting diode (OLED) technologies.

Researchers are looking for flexible alternatives to amorphous silicon, the semiconductor material used to construct the TFT and embed it on a glass substrate. Traditional manufacturing techniques require high temperatures that work on glass but would melt plastic substrates. Researchers are experimenting with "ink jet" printing of the transistors onto a thin polymer sheet. This requires moving from inorganic silicon to soluble, organic materials. HP is also testing imprint lithography, where a circuit pattern is pressed onto the polymer. Researchers at Palo Alto Research Center are also working with a stainless-steel foil substrate that can withstand high temperatures.

E-paper displays are called "bistable" because they can maintain an image when the power is turned off. Reflective displays don't require a backlight, as LCDs do, and can be read outdoors. The first generation has been used in signage, store-shelf price labels and e-book readers.

OLEDs emit their own light. They use more power than today's active-matrix LCDs but offer faster performance and richer colors. But manufacturing OLEDs on a flexible substrate presents challenges.

"OLEDs are probably further away than [e-paper] because OLEDs require a strong barrier against moisture, and plastic lets that right through," says Allen. Researchers have also had problems with display life spans, particularly with OLEDs that produce blue light, although some say that life spans have improved in the past few years.

Flexible displays are still under development or in the prototype stages for both e-paper and OLED technologies. "There are no displays that are dynamically flexible that are currently being used," says Allen. But she predicts that the market will ramp up from virtually nothing today to about US$338 million annually by 2013.

Among vendors of bistable displays, E Ink, is the best known. Its technology consists of a thin film atop a layer of electronic ink -- a series of black and white charged particles, or "pigments," suspended in a fluid that move up or down to create a black, white or gray image. So far, display manufacturers have used E Ink's technology to create e-book readers and what Mike McCreary, vice president of research and advanced development, calls "conformative displays" that are initially contoured to fit the shape of an object but remain rigid in the final product. Seiko has developed a watch display, for example, and Lexar Media has embedded a capacity meter for USB memory sticks using the technology. E Ink displays are also being used in a flexible electronic newspaper that's being tested in 200 households in Belgium.

Sipix Imaging, uses a similar technology to produce black, green or blue colors on a white background. Polymer Vision is working with Sipix and E Ink to create a rollable display prototype called the Readius. "We are enabling very small devices with large displays," says Edzer Huitema, program manager at Eindhoven, Netherlands-based Polymer Vision. The 5-in. pull-out display will offer 16 levels of gray. The device refreshes about once per second -- too slow for menu navigation or video but acceptable for portable navigation systems or as a Web news or e-mail reader. Huitema predicts that Sipix will ship the device in the first half of 2007, with color and touch-screen capabilities available by 2010. E Ink is working on a color filter for its technology that it expects to be ready in the same time frame.

NTera is developing a flexible, conformable bistable display based on its nanochromic technology. Instead of moving particles, NTera's RGB display technology determines colors based on each particle's charge. The design uses a 200-dpi passive- matrix transistor array, which is typically slower than active matrix because it updates each row on the screen instead of individual pixels, as active matrix does. However, NTera claims that its technology is fast enough to support video speeds for short intervals. The company says its partners will produce conformable displays early next year for uses such as side displays on cell phones or input tablets. "That is our primary focus," says Alain Briancon, chief technology officer at NTera.

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